Past Simple – Is ‘If I Knew You Were Ill, I Would Have Visited You’ Grammatically Correct?

The real discord in your second sentence

I've never known my grandmother […]

is not so much the fact that you no longer have an opportunity to know her, but the fact that you used never.

A more concordant phrasing would be

I never knew my grandmother, since she died before I was born.

Even if your grandmother were still alive (and, for example, you simply never met her), it would be strange to use have known with never. The reason is that have known is typically used to predicate of unbounded (not countable) groups. A context in which we use have known with never is in phrases like

I've never known anyone to turn down an offer of ice cream after dinner.

I have ridden a rollercoaster and now I am dizzy.

The only things wrong with your example sentence is that roller coaster is two words, and there should be a comma after them. This is to separate the two independent clauses. Otherwise, the grammar is fine.

Contextually, most folks would connect the two clauses as cause and effect. That is not the best way to word it, though. Potentially the only time you have ever ridden a roller coaster was 15 years ago, and you found yourself sidetracked mid-sentence (by your dizziness, perhaps). To force the interpretation you want, it would be better to say something like:

I just rode the roller coaster, and now I am dizzy.